Long Term Evolution (LTE) cellular communications systems use Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, as shown in FIG. 2, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
In LTE transmission using a single spatial layer, one transport block is transmitted to the receiver. When multiple spatial layers are used (such as Multiple Input Multiple Output (MIMO) transmission), two transport blocks are transmitted to the receiver. Since a transport block can be very large (e.g., up to 97896 bits for single spatial layer), a large transport block is divided into a number of code blocks that have suitable sizes for turbo encoding and decoding. For example, the transport block of size 97896 bits is divided into 16 code blocks of size 6144 bits each (including Cyclic Redundancy Check (CRC) bits per LTE specs TS 36.212).
In LTE systems, Hybrid Automatic Retransmission Request (HARQ) protocol is used to enhance transmission reliability. When an initial transmission is not received correctly by the receiver, the receiver stores the received signal in a soft buffer (implemented in a soft buffer memory, where “soft buffer memory” is physical/hardware memory utilized for the soft buffer) and signals to the transmitter of such unsuccessful transmission as illustrated in FIG. 3. The transmitter can then retransmit the information (referred to as the transport block in LTE specs) using the same channel coded bits or different channel coded bits. The receiver can then combine the retransmission signal with that stored in the soft buffer. Such combining of signals greatly enhances the reliability of the transmission. Incorrectly received coded data blocks may be stored as “soft bits” or soft values. These soft bits indicate what the receiver hypothesizes that the bit is and how certain the receiver is that this is a correct hypothesis. These soft bits can be combined with the retransmitted bits to calculate a more accurate hypothesis. These soft bits are stored in a soft buffer at the User Equipment (UE)/receiver so that, when the retransmitted block is received, the received values for the two blocks may be combined. Depending on the implementation, the receiver is only able to perform a certain number of soft bit reads from the soft buffer and soft bit writes to the soft buffer. As used herein, the total number of reads and writes that the receiver is capable of performing in the allotted time is referred to as the memory access bandwidth.
In the LTE system, the data transmission is protected by a rate 1/3 turbo code. To simplify signaling and operation complexity, a conceptual model referred to as a circular buffer is used in the LTE HARQ operations. This circular buffer model is illustrated in FIG. 4 for the case of single spatial layer transmission. The buffer consists of 32 columns of systematic bits followed by 64 columns of parity bits generated by the turbo encoder. The number of rows depends on the size of the transport block to be transmitted.
To simplify the signaling of what bits are transmitted to the receiver, four redundancy versions are defined. Each redundancy version is defined as the bits that can be read out of the circular buffer column-by-column starting from the head of a specific column in the circular buffer. The starting points of the four Redundancy Versions (RV): RV=0, 1, 2 and 3 are the heads of columns #2, #26, #50, and #74 (note the numbering of columns starts from 0). For a transmission using a specific redundancy version, the transmitter reads the bits starting from the start of the redundancy version until the necessary number of bits is obtained. If the reading reaches the end of the buffer and still more bits are needed, the reading of bits then resumes from the beginning of the buffer.
In the case of multi-spatial layer transmission, two transport blocks are transmitted in LTE. The circular buffer size is cut in half by discarding part of the parity bits. More specifically, the circular buffer model for LTE is illustrated in FIG. 5 for the case of multi-spatial layer transmission. In this case, the buffer consists of 32 columns of systematic bits followed by 16 columns of parity bits generated by the turbo encoder. The starting points of the four redundancy versions RV=0, 1, 2 and 3 are the heads of columns #2, #14, #26 and #38 (note the numbering of columns starts from 0).
It can be appreciated by one skilled in the art that it is generally advantageous to perform retransmission that carries more bits that have not been transmitted in previous transmission attempts. For instance, in a so-called Chase Combining protocol, the transmitter sends the initial transmission using RV=0 and resends subsequent retransmissions also using RV=0. That is, in Chase Combining, every retransmission contains the same information (data and parity bits). The receiver uses maximum-ratio combining to combine the received bits with the same bits from previous transmissions. Because all transmissions are identical, Chase Combining can be seen as additional repetition coding. That is, every retransmission adds extra energy to the received transmission through an increased Eb/N0 (the energy per bit to noise power spectral density ratio).
Such a simple HARQ protocol as Chase Combining mostly provides benefits from combining the signal energy from the transmissions resulting in, for example, a 3 dB gain for 2 transmissions and 4.8 dB for 3 transmissions. On the other hand, in a so-called Incremental Redundancy protocol, the transmitter picks a redundancy version that shares the lowest number of bits, such as the redundancy version that was used in the initial transmissions. That is, when using Incremental Redundancy, every retransmission contains different information than the previous transmission. Multiple sets of coded bits are generated, each representing the same set of information bits. The retransmission typically uses a different set of coded bits than the previous transmission, with different redundancy versions generated by puncturing the encoder output. Thus, at every retransmission the receiver gains extra information. This HARQ protocol provides both the energy gain as well as additional coding gains. Using the highest rate transmission using 256 Quadrature Amplitude Modulation (QAM) as an example, 8.4 decibel (dB) gains can be obtained after two transmissions, and 11.3 dB gains can be obtained after three transmissions.
The LTE Rel-10 standard supports bandwidths larger than 20 megahertz (MHz). One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier wider than 20 MHz should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments, it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to ensure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least could possibly have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 6. A CA-capable UE is assigned a Primary Cell (PCell) which is always activated, and one or more Secondary Cells (SCells) which may be activated or deactivated dynamically.
In Rel-13, LAA (Licensed-Assisted Access) has attracted a lot of interest in extending the LTE carrier aggregation feature towards capturing the full breadth of opportunities of unlicensed spectrum in the 5 GHz band. A Wireless Local Area Network (WLAN) operating in the 5 gigahertz (GHz) band nowadays already supports 80 MHz in the field, and 160 MHz is to follow in Wave 2 deployment of Institute of Electrical and Electronics Engineers (IEEE) 802.11ac. There are also other frequency bands, such as 3.5 GHz, where aggregation of more than one carrier on the same band is possible, in addition to the bands already widely in use for LTE. Enabling the utilization of bandwidths for LTE in combination with LAA similar to bandwidths used for IEEE 802.11ac Wave 2 will lead to proposals for extending the carrier aggregation framework to support more than five carriers. The extension of the CA framework beyond five carriers was approved to be one work item for LTE Rel-13. The objective is to support up to thirty two carriers in both Uplink (UL) and Downlink (DL).
To support up to 32 carriers in DL, the Uplink Control Information (UCI) feedback, e.g. HARQ-ACK bits, will increase significantly. For each DL subframe, there are 1 or 2 HARQ-ACK bits per carrier depending on whether spatial multiplexing is supported or not. Hence, for Frequency-Division Duplex (FDD), there can be up to 64 HARQ-ACK bits if there are 32 DL carriers. The number of HARQ-ACK bits for Time-Division Duplex (TDD) is even larger, potentially as high as hundreds of bits depending on the TDD configuration. Therefore, a new Physical Uplink Control Channel (PUCCH) format(s) supporting larger payload is necessary. Similarly, the piggyback of the increased number of UCI bits also motivates the enhancements on UCI feedback on Physical Uplink Shared Channel (PUSCH).
In the LTE specification, each UE is required to store a specific number of received soft bits in its soft buffer. To support high data rate communications, high read and write bandwidths are needed for such soft buffer to and from the baseband processor and the turbo decoder. It has hence been a general practice to incorporate the soft buffer in the same chip with the baseband processor and the turbo decoder.
It has been suggested that the traditional solution of collocating the soft buffer and the baseband processor may not be an economically viable or even technically feasible solution for supporting a large number of carriers. It has further been suggested to adopt off-chip memory. Such a solution would have only limited bandwidth to read and write the soft bits.
Using the single-spatial layer transmission case as a non-limiting example, a receiver will store the soft bits corresponding to RV=0 in the off-chip soft buffer. The bandwidth issue is most limiting when the transmission is using the highest order modulation and the highest coding rate allowed in the LTE specs. This corresponds to 256 QAM Modulation and Coding Scheme (MCS) 27 with code rate r=0.9035. At such code rate, approximately [32/0.9035]=36 columns of soft bits are stored to the soft buffer. When a retransmission using the same RV=0 is received by the receiver, the receiver shall read out the previously stored soft bits and combine them with the newly received soft bits for decoding. If the decoding still fails, the receiver shall write the combined soft bits back to the soft buffer.
As illustrated in FIG. 7, the receiver hardware shall be designed to support enough soft buffer access bandwidth to accomplish the following two sets memory read-writes when the highest order modulation and the highest coding rate are used in the transmissions:                36 cols (#2-#37) read from memory to combine with new soft bits        After decoding fails, 36 cols (#2-#37) of new combined soft bits are written back to memory        
This memory access bandwidth is proportional to read-writes of 72 columns per turbo code block for the highest MCS transmission. For a receiver designed to just meet such a minimum memory access bandwidth requirement, it may not be able to read the entirety of the stored soft bits from the off-chip memory for soft combining such as those illustrated in FIG. 8 for the single-spatial layer transmission case where two retransmissions fail. Such situation requires memory access bandwidth proportional to read-writes of 108 columns per turbo code block for the highest MCS transmission, which is higher than the designed bandwidth proportional to read-writes of 72 columns.
Further consider the case of multi-spatial layer transmission where two transport blocks are transmitted. For each code block for each transport block, the memory access bandwidth is proportional to read-writes of 72 columns per turbo code block for the highest MCS transmission as illustrated in FIG. 9. Since the two transport blocks may both fail, the receiver should be designed to support memory access bandwidth proportional to read-writes of 2×72=144 columns per turbo code block for the highest MCS transmission.
Similarly, in the case of multi-spatial layer transmission illustrated in FIG. 10 where two retransmissions fail, the required memory access bandwidth is proportional to read-writes of 2×84=168 columns per turbo code block for the highest MCS transmission, which is higher than the designed bandwidth proportional to read-writes of 144 columns.
To overcome such bandwidth limitation, it has been suggested to restrict the HARQ protocol operations to using mostly the same redundancy version as the initial transmissions (i.e., the Chase Combining protocol). However, such a solution severely limits the system performance, as there are large performance differences between Chase Combining and Incremental Redundancy protocols as discussed above. As such, systems and methods are needed for soft buffer handling with limited memory access bandwidth.